The invention relates to a reinforcing strip comprising steel and adapted for the reinforcement of the belt of rubber vehicle tires, and having a significantly higher bending stiffness in the plane of the strip than in the longitudinal plane (i.e. a plane comprising the longitudinal axis of the strip), perpendicular to the plane of the strip. The strip according to the invention must be preferably used for the reinforcement of vehicle tires the reinforcement plies of which have at the meridian plane a radius of curvature of at least 1500 mm. Such strip can be in the form of a continuous long strip that is, or can be wound on a spool, or in the form of one or more separate ends of a length which preferably ranges in the order of magnitude of about 30 cm, e.g. in the range from 15 cm to 50 cm.
The mention that the strips are of the type that are adapted for the reinforcement of the belt of rubber vehicle tires does not limit its possible use in other applications such as the reinforcement of elastomeric articles in general. The mention means only that the strips have the necessary characteristics for such use, which are: a steel cross-section of the order of magnitude ranging between 0.05 mm.sup.2 and 2 mm.sup.2, preferably in the range between, 0.150 mm.sup.2 and 1 mm.sup.2, a tensile strength of the reinforcing steel of more than 2200 N/mm.sup.2, preferably more than 2500 N/mm.sup.2, an elongation at break of more than 1.5%, the reinforcing steel being covered with a rubber adherable coating, such as e.g. a metallic coating of brass.
It is already known, e.g. from U.S. Pat. No. 3,794,097, to form the belt ply of rubber vehicle tires, by laying short ends of nearly rectangular steel strips, instead of conventional steel cord, in a parallel disposition side by side with interstices between adjacent strips, filled with rubber.
A first advantage with respect to the use of conventional steel cords is, that the same amount of reinforcing steel can be laid in a thinner layer so as to obtain thinner and lighter plies. A second and important advantage is, that the strips have a much higher bending stiffness in the plane of the belt, and this reduces the deformation and heat generation under alternating shearing stresses in that plane, whilst maintaining good flexibility in any plane perpendicular to the belt.
The existing steel strips present however the important drawback, with respect to the use of conventional steel cord, of poor mechanical properties, especially tensile strength and fatigue endurance, due to the method in which they have to be made. Known methods are: slitting steel sheet or flat rolling of round wire. Slitting steel sheet produces sharp edges where stresses and fatigue crack initiation are concentrated. Flat cold rolling of round wire doesnot yield a high tensile strength level, because the rolling must be stopped far before such high level is reached in order to keep sufficient ductility for the subsequent rolling operation, in which the tensile strength level drops again. Due to the fact that in general the obtained fatigue resistance will be about 33% of the obtainable tensile strength, and that the strip in general shows some delamination bursts due to rolling, it will be difficult to reach a fatigue resistance of 600 Newton/mm.sup.2, whereas the new obtainable fatigue resistance of conventional high-tensile steel cord lies about twice this amount.
Attempts have been made to improve the mechanical properties of the steel strips by the use of an appropriate heat treatment, such as disclosed in U.S. Pat. No. 4,017,338 and 4,142,920. This results however into additional manufacturing costs and the mechanical properties as disclosed are still far from those of conventional steel cord. Among other things, the delamination bursts can indeed not be repaired by any heat treatment.